This invention relates to a process for making encapsulated circuit boards and products made by said process.
Considerable prior art discloses thermal curing of adhesives used in the manufacture of conventional printed circuit boards (e.g., see U.S. Pat. Nos. 4,115,185 and 4,180,608). Ultraviolet (UV) radiation has also been used to cure the adhesives employed in printed circuit board manufacture (e.g., see U.S. Pat. No. 4,305,854). In the manufacture of conventional printed circuit boards, circuit patterns are created by a sequence of steps involving deposition of photoresist, masking, exposure, etching of unexposed (or exposed) resist, and metal deposition. For multi-layered boards, these steps must be repeated to build up successive layers. As circuit density increases, precise registration of successive layers becomes more critical and control of the processing steps becomes more difficult. This greatly increases the probability of short and open circuits, impairing quality and reliability.
To overcome these and other difficulties, encapsulated circuit boards have been proposed (e.g., see U.S. Pat. Nos. 3,646,572; 3,674,914; and 3,674,602). The typical process employed in their manufacture is to dispose a plurality of insulation-coated wires in preselectable positions in a thermo-setting resin adhesive which is not cured and remains pliable after application. This undesirably enables the wires to shift from their preselected positions and lose critical registration until heat and pressure is applied to fully cure the thermo-setting resin and permanently bond the wires to a substrate.
In the above-referenced related Varker application an improved process for fabricating an encapsulated circuit board is disclosed and claimed. According to his process, a plurality of wires is pressed into an adhesive that is thermally cured to inhibit movement of the wires when so positioned. This application also suggests that, under certain circumstances (e.g., where a large number of wires are to be carried by the board), it may be desirable to rigidly position the wires in the X and Y directions but permit limited mobility perpendicular thereto (i.e., in Z direction) by use of a two-part epoxy adhesive to enable thermal curing of the adhesive everywhere except under the wires. However, such selective curing was found difficult to achieve.
There is a need for a simpler and less expensive process than thermal curing of a two-part epoxy adhesive to achieve dimensional stability of the wires in the X and Y directions, while permitting limited mobility and flexibility of the adhesive (and hence movement of the wires) in the Z direction.